In public he was the lean and ruthless face of American military outsourcing in Iraq. Erik Prince, as founder of the Blackwater security company, packed a mobile phone on one hip and a handgun on the other as he flew in and out of the world’s troublespots co-ordinating protection teams for American VIPs — and handling the backlash when his employees were accused of shooting dead 17 Iraqi civilians at a Baghdad crossroads in 2007.

In private, he was a CIA operative, with his own file as a “vetted asset” at the agency’s headquarters, and a mission to build “a unilateral, unattributable capability” to hunt down and kill al-Qaeda militants for the US Government wherever they could be found.

These claims, made by Mr Prince and supported by others who knew of his activities, form part of a potentially explosive investigation into the life of America’s best-known mercenary.

Mr Prince, aside from his work in Iraq, set up America’s closest forward operating base to the Pakistani border in Afghanistan, and helped to train a CIA assassination team that hunted an alleged senior al-Qaeda financier in Germany, and included A. Q. Khan, a Pakistani nuclear scientist, on its list of targets, according to Vanity Fair magazine.